The production of steel normally comprises the steps of converting iron ore to pig iron using a blast furnace, and thereafter converting the pig iron into steel using an open hearth furnace or a converter. Such a traditional method requires large amounts of energy and large-scale equipment, and has a high cost. Therefore, for a small-scale steel-making, a method comprising the steps of directly converting iron ore into raw materials used in the steel-making furnace, and converting the raw material into steel using an electric furnace and the like has been used. With respect to direct steel making process, a direct reduction process has been used to convert iron ore into reduced iron. However, the reduced iron produced by the direct reduction process is highly reactive and reacts with oxygen in the air to generate heat. Therefore, it is necessary to seal the reduced iron with an inert gas, or by some other measures, during transportation and storage of the reduced iron. Accordingly, iron carbide (Fe.sub.3 C) containing a comparatively high iron (Fe) content, and which has a low reaction activity and can be easily transported and stored, has recently been used as the iron-containing material for steel making in an electric furnace and the like.
Furthermore, an iron-making or steel-making material containing iron carbide as the main component is not only easy to be transported and stored, but also has the advantage that the carbon combined with iron element can be used as a source of energy in an iron-making or steel-making furnace, and can be used as a source to generate microbubbles which reduce nitrogen in the steel-making bath. Therefore, raw materials for iron making or steel making containing iron carbide as the main component recently have attracted special interest.
According to a conventional method for producing iron carbide, a fine iron ore is charged into a fluidized bed reactor or the like, and is caused to react with a gas mixture comprising a reducing gas (e. g., hydrogen gas) and a carburizing gas (e. g., methane gas and the like) at a predetermined temperature. Thus, iron oxides (e. g., hematite (Fe.sub.2 O.sub.3), magnetite (Fe.sub.3 O.sub.4), wustite (FeO)) in iron ore are reduced and carburized in a single process (which means a process performed by simultaneously introducing a reducing gas and a carburizing gas to a single reactor). This reaction is performed by the following overall reaction formula. EQU 3Fe.sub.2 O.sub.3 +5H.sub.2 +2CH.sub.4.fwdarw.2Fe.sub.3 C+9H.sub.2 O
The prior art in the field of the present invention has been described, for example, in the publication of the Japanese translation of International Patent Application No. 6-501983, for example.
In order to easily understand the present invention, an example of an apparatus for producing iron carbide according to the prior art will be described below. For example, an apparatus shown in FIG. 1 has been known. With reference to FIG. 1, the reference number 1 denotes a fluidized bed reactor. Fluidized bed reactor 1 has a bottom part to which a line 2 for supplying reaction gases (a reducing gas and a carburizing gas) is connected, and a top part to which a line 3 for discharging the gas after reaction is connected. The reference number 4 denotes a preheating furnace. A fine iron ore fed to preheating furnace 4 is subjected to a preheating treatment for a predetermined time in preheating furnace 4. Then, the preheated iron ore is fed into fluidized bed reactor 1 through a line 5, and is subjected to a reducing and carburizing reaction for a predetermined time at a predetermined reaction temperature and reaction pressure in fluidized bed reactor 1. Thus, iron carbide product is discharged from a line 6.
In the case where a particle size distribution of the iron ore is wide, it is difficult to proceed the reaction efficiently. The reason is as follows. In order to proceed the reaction efficiently, it is preferable that a velocity of a fluidized gas should be comparatively increased in fluidized bed reactor 1 if major particle size of the iron ore is large (coarse) but fine ores should be blown off, and that the velocity of the fluidized gas should be comparatively decreased in fluidized bed reactor 1 if major particle size of the iron ore is small (fine) but coarse ores should not be fluidized. There are preferable process conditions depending on respective particle sizes. Furthermore, a moving bed reactor is preferable for the iron ore having a large particle size. A gas for proceeding the reaction can easily pass through a gap in the large uniform particle size. An increase in the flow rate of the fluidized gas for fluidization causes generation of fine-sized iron ore by a further friction of particles and is disadvantageous to a yield of iron ores.
As indicated in the overall reaction formula, a particle of Fe.sub.2 O.sub.3 is converted into a particle of Fe.sub.3 C having about 3/4 of an original weight. Furthermore, the fine iron ores rub against each other during fluidization so that their particle sizes are gradually reduced. Taking it into consideration that the weight of the fluidized material (fine iron ore) is gradually reduced as the reaction proceeds, it is preferable that the velocity of the reaction gas to be supplied to the fluidized bed reactor should be comparatively increased in the former half of the reaction and be comparatively decreased in the latter half of the reaction in order to proceed the reaction efficiently. Since there are proper process conditions according to the progress of the reaction, it is not preferable that the reducing reaction and the carburizing reaction should be performed under the same process conditions in the fluidized bed reactor.
In consideration of the above-mentioned problems of the prior art, it is an object of the present invention to provide a method for efficiently producing iron carbide depending on a particle size of an iron-containing material or the progress of reaction.